A total of 15 candidate genes associated with both RA and IBD were identified after the intersection of genes in the abovementioned target modules: CXCL9, CCL18, CXCL10, S100A9, MMP9, RARRES3, S100A8, FCN1, ISG20, LILRB2, IDO1, CD19, CIITA, SIRPG, and DUOX2 (Fig. 3J). Here, CIITA is linked to rheumatoid arthritis.